The present invention relates to metallic glass alloys for use in electronic article surveillance systems.
Metallic glass alloys (amorphous metal alloys or metallic glasses) have been disclosed in U.S. Pat. No. 3,856,513, issued Dec. 24, 1974 to H. S. Chen et al. (the xe2x80x9c""513xe2x80x9d Patent) These alloys include compositions having the formula MaYbZc, where M is a metal selected from the group consisting of iron, nickel, cobalt, vanadium and chromium; Y is an element selected from the group consisting of phosphorus, boron and carbon; Z is an element selected from the group consisting of aluminum, silicon, tin, germanium, indium, antimony and beryllium; xe2x80x9caxe2x80x9d ranges from about 60 to 90 atom percent; xe2x80x9cbxe2x80x9d ranges from about 10 to 30 atom percent; and xe2x80x9ccxe2x80x9d ranges from about 0.1 to 15 atom percent. Also disclosed are metallic glass wires having the formula TiXj, where T is at least one transition metal and X is an element selected from the group consisting of phosphorus, boron, carbon, aluminum, silicon, tin, germanium, indium, antimony and beryllium, xe2x80x9cixe2x80x9d ranges from about 70 to 87 atom percent and xe2x80x9cjxe2x80x9d ranges from about 13 to 30 atom percent. Such materials are conveniently prepared by rapid quenching from the melt using processing techniques that are now well known in the art.
Metallic glass alloys substantially lack any long-range atomic order and are characterized by x-ray diffraction patterns consisting of diffuse (broad) intensity maxima, qualitatively similar to the diffraction patterns observed for liquids or inorganic oxide glasses. However, upon heating to a sufficiently high temperature, they begin to crystallize with evolution of the heat of crystallization; correspondingly, the x-ray diffraction pattern thereby begins to change from that observed for amorphous materials to that observed for crystalline materials. Consequently, metallic alloys in the glassy form are in a metastable state. This metastable state of the alloy offers significant advantages over the crystalline form of the alloy, particularly with respect to the mechanical and magnetic properties of the alloy.
Use of metallic glasses in magnetic applications has been disclosed in the ""513 Patent. However, certain combinations of magnetic properties are needed to realize magnetic components required in modern electronics technology. For example, U.S. Pat. No 5,284,528 issued Feb. 8, 1994 to Hasegawa et al., addresses such a need. One of the important magnetic properties that affects the performance of a magnetic component used in electrical or electronic devices is called magnetic anisotropy. Magnetic materials are, in general, magnetically anisotropic and the origin of the magnetic anisotropy differs from material to material. In crystalline magnetic materials, one of the crystallographic axes could coincide with the direction of magnetic anisotropy. This magnetically anisotropic direction then becomes the magnetic easy direction in the sense that the magnetization prefers to lie along this direction. Since there are no well-defined crystallographic axes in metallic glass alloys, magnetic anisotropy could be considerably reduced in these materials. This is one of the reasons that metallic glass alloys tend to be magnetically soft, which makes them useful in many magnetic applications. The other important magnetic property is called magnetostriction, which is defined as a fractional change in physical dimension of a magnetic material when the material is magnetized from the demagnetized state. Thus, magnetostriction of a magnetic material is a function of applied magnetic field. From a practical standpoint, the term xe2x80x9csaturation magnetostrictionxe2x80x9d (xcexs) is often used. The quantity xcexs is defined as the fractional change in length that occurs in a magnetic material when magnetized along its length direction from the demagnetized to the magnetically saturated state. The value of magnetostriction is thus a dimensionless quantity and is given conventionally in units of microstrain (i.e., a fractional change in length, usually parts per million or ppm).
Magnetic alloys of low magnetostriction are desirable for the following reasons:
1. Soft magnetic properties characterized by low coercivity, high permeability, etc. are generally obtained when both the saturation magnetostriction and the magnetic anisotropy of the material become small. Such alloys are suitable for various soft magnetic applications, especially at high frequencies.
2. When magnetostriction is low and preferably zero, magnetic properties of such near-zero magntostrictive materials are insensitive to mechanical strain. When this is the case, there is little need for stress-relief annealing after winding, punching or other physical handling needed to form a device from such material. In contrast, magnetic properties of stress-sensitive materials are considerably degraded by even small elastic stresses. Such materials must be carefully annealed after the final forming step.
3. When magnetostriction is near zero, a magnetic material under ac excitation shows a small magnetic loss due to a low coercivity and to reduced energy loss by reduced magneto-mechanical coupling via magnetostriction. Thus, near-zero magnetostrictive magnetic materials are useful where low magnetic loss and high permeability are required. Near-zero magnetostrictive material is, therefore, desirable when it is used as a marker in an article surveillance system based on utilizing higher harmonics generated by the marker. U.S. Pat. No. 4,553,136 issued on Nov. 12, 1985 to Anderson et al addresses such a case.
There are three well-known crystalline alloys of zero or near-zero magnetostriction: Nickel-iron alloys containing approximately 80 atom percent nickel (e.g. xe2x80x9c80 Nickel Permalloysxe2x80x9d); cobalt-iron alloys containing approximately 90 atom percent cobalt; and iron-silicon alloys containing approximately 6.5 wt. percent silicon. Of these alloys, permalloys have been used more widely than the others because they can be tailored to achieve both zero magnetostriction and low magnetic anisotropy. However, these alloys are prone to be sensitive to mechanical shock, which limits their applications. Cobalt-iron alloys do not provide excellent soft magnetic properties due to their strong negative magnetocrystalline anisotropy. Although some improvements have been made recently in producing iron-based crystalline alloys containing 6.5% silicon [J. Appl. Phys. Vol. 64, p.5367 (1988)], wide acceptance of them as a technologically competitive material is yet to be seen.
As mentioned above, magnetocrystalline anisotropy is effectively absent in metallic glass alloys due to the absence of crystal structures. It is, therefore, desirable to seek glassy metals with zero magnetostriction. The above mentioned chemical compositions which led to zero or near-magnetostriction in crystalline alloys were thought to give some clues to this effort. The results, however, were disappointing. To this date, only Co-rich and Coxe2x80x94Ni-based alloys with small amount of iron have shown zero or near-zero magnetostriction in glassy states. Examples for these alloys have been reported for Co72Fe3P16B6Al3 (AIP Conference Proceedings, No. 24, pp.745-746 (1975)) and Co31.2Fe7.8Ni39.0B14Si8 (Proceedings of 3rd International Conference on Rapidly Quenched Metals, p.183 (1979)). Co-rich metallic glass alloys with near-zero magnetostriction are commercially available under the trade names of METGLAS(copyright) alloys 2705M and 2714A (Honeywell International Inc) and VITROVAC(copyright)6025 and 6030 (Vacuumschmelze GmbH). These alloys have been used in various magnetic components operated at high frequencies. Although the above-mentioned Coxe2x80x94Ni based alloy show near-zero magnetostriction, this and similar alloys have never been widely commercialized. Only one alloy (VITROVAC 6006) based on Coxe2x80x94Ni-based metallic glass alloys has been commercially available for anti-theft marker application (U.S. Pat. No. 5,037,494). These alloys have saturation magnetic induction below 0.5 T and have limited applications. For example, to compensate the low level of saturation magnetic induction of these alloys, a thin and narrow ribbon is required to achieve a workable anti-theft or electronic article surveillance marker. In addition, this ribbon has to be heat-treated in a magnetic field to realize the desired property as a magnetic marker in electronic article surveillance systems. Such heat-treatment sometimes results in a brittle ribbon, which makes it difficult to cut the ribbon to a desired length for an electronic article surveillance marker and, in turn, leads to a fragile marker in actual operation. Clearly desirable are new magnetic metallic glass alloys based on Co and Ni that are magnetically more versatile and mechanically more ductile than the existing alloy for applications in electronic article surveillance systems.
In accordance with the invention, there is provided a magnetic alloy that is at least 70% glassy and which has a low magnetostriction. The metallic glass alloy has the composition CoaNibFecMdBeSifCg where M is at least one element selected from the group consisting of Cr, Mo, Mn and Nb; xe2x80x9ca-gxe2x80x9d are in atom percent and the sum of xe2x80x9ca-gxe2x80x9d equals 100; xe2x80x9caxe2x80x9d ranges from about 25 to about 60; xe2x80x9cbxe2x80x9d ranges from about 5 to about 45; xe2x80x9ccxe2x80x9d ranges from about 6 to about 12; xe2x80x9cdxe2x80x9d ranges from 0 to about 3; xe2x80x9cexe2x80x9d ranges from about 5 to about 25; xe2x80x9cfxe2x80x9d ranges from 0 to about 15; and xe2x80x9cgxe2x80x9d ranges from 0 to about 6. The metallic glass alloy has a value of the saturation magnetostriction ranging from about xe2x88x923 to +3 ppm. The metallic glass alloy is cast by rapid solidification from the melt into ribbon or sheet or wire form. Depending on the need, the metallic glass alloy is heat-treated (annealed) with or without a magnetic field below its crystallization temperature. The metallic glass alloy thus prepared is cut into a desired strip which preferably has a non-linear Bxe2x80x94H behavior when measured along the strip""s length direction. The strip, whether it is heat-treated or not, is ductile in order to realize a workable magnetic marker for electronic article surveillance applications.